The present invention relates to an injection-molding apparatus and a method of injection-molding, and more particularly, to an injection-molding apparatus for forming a molded product from a molding material comprising a thermoplastic resin and an inorganic filler, which is capable of not only reducing costs for production facilities and shortening an injection-molding cycle time, thereby achieving a high productivity, but also effectively preventing deformation of the molded product, and a method for injection-molding a molding material comprising a thermoplastic resin and an inorganic filler using such an apparatus.
The injection-molding apparatus and method of injection-molding according to the present invention are useful for the production of such articles required to have a high dimensional accuracy, for example, electronic parts such as a magnet roll or the like.
In general, molded products such as magnet rolls have been produced by subjecting a molding material comprising a thermoplastic resin such as a polyamide resin or the like and an inorganic filler such as ferrite particles or iron oxide particles to an injection-molding process. A molding cycle of such an injection-molding cycle comprises steps of clamping, injecting, dwelling, cooling, mold-opening and removing the molded product from the mold. In the production of magnet roll, etc., these molded products have been required to have a high dimensional accuracy. On the other hand, various methods for reducing the time required for conducting the above-mentioned steps, especially a cooling step, have been studied in order to achieve a high productivity.
Meanwhile, in the above-mentioned production, if the cooling time in the mold is shortened, the molded product cannot be sufficiently cooled in the mold before its removal. This causes internal strain of the molded product generated upon injection-molding to be released, so that the molded product undergoes deformation such as warpage or bend. In addition, there have been proposed a method of shortening the cooling time by lowering the temperature of the mold. However, in the case where the temperature of the mold is too low, a resin flowing on an inner surface of the mold is abruptly solidified upon injection thereof, so that generation of strain in the molded product is further promoted.
Therefore, there has been recently proposed a method of shortening a molding cycle time, which method is called "OCI (Outside Cooling Injection)" (refer to the magazine "PLASTICS" Vol. 45, No. 45, pp. 37-41(1994) and Japanese Patent Application Laid-open (KOKAI) No. 7-9498(1995)). In the OCI method, there have been used two molds (A and B), and a resin is first injected into the mold A and then into the mold B. That is, (1) after completion of the injection at the mold A, the resin is injection-molded in the mold B. (2) During the injection in the mold B, a molded product obtained at the mold A is cooled and removed therefrom. (3) After completion of the injection at the mold B, the resin is injected again into a vacant cavity of the mold A. According to the OCI method, the above-mentioned molding operations are repeated alternately at the molds A and B, thereby ensuring a sufficient cooling time and enhancing a productivity of molded products.
However, in the OCI method, at least two identical molds and a large-size injection unit for injecting the resin to the respective molds are necessary, thereby considerably increasing costs for production facilities. The OCI method actually requires a high cost exceeding an upper limit of desired production cost, so that it becomes extremely difficult to adopt the OCI method.
As a result of the present inventors' earnest studies for solving the above-mentioned problems in prior arts, it has been found that in an injection-molding apparatus comprising an injection machine and a mold unit, by providing a holding means comprising a plural of chucks, disposed adjacent to the mold unit having a cavity (or cavities) for removing the molded product from the mold unit, wherein the ratio of the number of the chuck to the number of the cavity of the mold unit is set to a value not less than the ratio of operation time of each chuck to operation time of the mold unit, not only the reduction of costs for production facilities and the shortening of an injection-molding cycle time can be achieved, but also the effective prevention of the deformation of the molded product can be attained. On the basis of the above-mentioned finding, the present invention has been achieved.